The present invention relates generally to ink jet printing systems and deals more particularly with a hydraulic servomechanism for controlling the pressure of writing fluid supplied to an ink jet printer heat.
In an ink jet printer a receiving surface on which a graphic is to be created is moved relative to one or more ink jet printer heads in a line scanning fashion. As each printer head moves along a scan line, it moves past a succession of points on the line in relation to each of which the printer head may eject a drop of writing fluid such as ink, paint, pigmented ink, etc., which lands on and prints a dot at the position. In one type of printer head the head is actuated for each potential print point on the scan line, to eject a drop of ink for each such position, and then the drop is electrostatically controlled during its flight from the printer head to the receiving surface to either direct it onto the receiving surface or away from the receiving surface depending on whether the scan line point in question is to be printed or not. In such a printer head the actuation frequency, or the time between successive actuations is dependent on the speed of the printer head along the scan line.
In another type of printer head, referred to as a "drop-on-demand" printer head, as the printer head is moved along a scan line it is actuated to produce a drop of ink only for those potentional print positions along the scan line onto which the printing of dots is wanted. Therefore, the amount of time elapsing between successive actuations is dependent not only on the speed of the printer head relative to a receiving surface, but also on the pattern in accordance to which dots are to be printed along the scan line.
In either type of printer head described above, after a drop is ejected from the head, it travels for some distance in free flight from the printer head to the receiving surface along a trajectory path dependent on the velocity at which the drop is ejected. Changes in the ejected velocity therefore change the location at which a drop strikes the receiving surface and are quite undesireable. Also, for good printing all ejected drops should be of substantially the same volume so that all dots printed on the receiving surface by the separate drops are of substantially consistent size.
Because of fluid and mechanical dynamics involved in the actuation of a printer head, including resonances and other phenomena the ejected drop velocity and volume varies widely in many printer heads with the changes in the actuation frequency or the time elapsing between successive actuations. This may be somewhat troublesome in the use of electrostatically deflected printer heads in the cases where the printer head is moved at different speeds relative to the receiving surface. It is, however, particularly troublesome in the case of "drop-on-demand" printer heads in which the inherent operation of the printer involves a wide range in the elapsed time occuring between successive pulses. That is, while scanning a line during one portion of the line, the printer head may be actuated to print the dot at every potential print point, in which case a very short elapsed time occurs between successive actuations, and along other portions of the line, the printer head may be acutated to print the dot only at some ocassional potential print points in which case the time elapsing between successive actuations is considerably lengthened. In the case of large volume "drop-on-demand" ink jet printer heads which produce dots having a printed size ranging from 0.02 to 0.05 inches or larger, the fluid and mechanical dynamics of the printer head tend to limit the rate at which the dots may be ejected with a constant velocity and consistent volume. In U.S. Pat. No. 4,562,445, issued Dec. 31, 1985 to Rich and entitled METHOD AND APPARATUS FOR DRIVING AN INK JET PRINTER, and assigned to the same assignee as the present invention, a driving circuit for an ink jet printer head is disclosed for causing the printer head to eject the drops at a constant velocity despite changes in the time elapsing between successive actuations.
In another co-pending patent application entitled "Apparatus and Method for Dynamically Varying the Pressure of Writing Fluid Supplied to an Ink Jet Printer Head" Ser. No. 820,520 filed concurrently herewith and assigned to the same assignee as the present invention, now U.S. Pat. No. 4,651,161 a system for controlling the pressure of writing fluid supplied to an ink head is disclosed wherein the pressure of the ink supplied to the head is adjusted to compensate for pressure drops within the piezo cavity of the ink jet head associated with the higher actuation rates to cause the head to eject drops of consistent volume at the higher actuation rates. The writing fluid pressure control system disclosed in the above-identified patent application utilizes a pressure control mechanism to adjust the pressure and includes a variable volume fluid chamber. The chamber volume is increased or decreased in accordance with a dot rate command signal generated from a controller to adjust the pressure of the ink contained in the chamber. The ink in turn is supplied to the ink jet head at a desired pressure to insure that the pressure of the ink in the piezo cavity is at the desired pressure during actuation so that ejected drops are of a consistent volume over a large range of actuation frequencies.
An object of the present invention is, therefore, to provide a hydraulic servomechanism for controlling the pressure of writing fluid supplied to an ink jet printer head to insure that the printer head ejects drops of a substantially consistent volume over a wide range of actuation frequencies.
Another object of the present invention is to provide a hydraulic servomechanism that is responsive to a wide dynamic range of changes in ink jet printer head actuation frequencies.
A further object of the present invention is to provide a hydraulic servomechanism for use with a large scale ink jet printing system that is reliable, less costly and less complex than previously used pressure control mechanisms for controlling the pressure of writing fluid supplied to an ink jet printer head.